The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio at a minimum transmission ratio.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
When starting the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced at a speed which is determined by line pressure, the pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio of the transmission. In such a system, the speed of changing of transmission ratio up to a desired transmission ratio can not be controlled in accordance with driving conditions. Accordingly, hunting or overshooting of the transmission ratio occurs, which decreases the driveability of the vehicle.
Japanese Patent Laid Open 59-159456 discloses a system provided with a first valve for changing the direction of the transmission ratio change and a second valve for controlling the transmission ratio changing speed. By controlling the spool of the second valve, the transmission ratio changing speed is controlled. However, the system is complicated in construction, since two control valves are provided in addition to the conventional system.
Japanese Patent Laid Open 59-217048 shows a system which operates to vary a desired transmission ratio in accordance with the deviation of an actual transmission ratio from the desired ratio. However, such a system causes overshooting of the control operation.